Anti-skid brake control system with operation control for a pressure reduction fluid pump in hydraulic brake circuit

ABSTRACT

An anti-skid brake control system includes a fluid pump which facilitates brake release when wheel deceleration, as indicated in a derived wheel acceleration value, exceeds a predetermined level. Since at low vehicle speeds, the sensors used to derive the wheel acceleration value are of unsatisfactory accuracy, the wheel acceleration value itself may be inaccurate. Accordingly, at low vehicle speeds, the fluid pump is effectively disabled by replacing the derived wheel acceleration value with a fixed value greater than the predetermined level.

BACKGROUND OF THE INVENTION

The present invention relates generally to an anti-skid brake controlsystem for an automotive vehicle, which includes a pressure reductionfluid pump operative in response to a braking pressure releasingcommand. More particularly, the invention relates to a control systemfor controlling operation of a power servo system for reducing fluidpressure in a wheel cylinder while the anti-skid control system is inoperation in a pressure releasing mode.

Published Japanese Pat. No. 49-32494, published on Aug. 30, 1974discloses an anti-lock brake control system having a power servo systemwhich recirculates pressurized fluid in a wheel cylinder when released.The power servo system comprises an electric motor and a fluid pumpdriven by the electric motor. In such anti-skid control systems, it isconsidered effective to drive the fluid pump while the anti-lock oranti-skid control system is active.

In general, the anti-lock systems become active when wheel decelerationexceeds a set threshold. Assuming the electric motor with the fluid pumpstarts operating whenever the anti-lock system is active, a problem willarise at relatively low vehicle speeds, e.g. lower than 10 km/h. In suchcases, the accuracy of the wheel speed sensor is lowered, increasingsystematic error in the detected wheel speed. Since the wheeldeceleration is derived from the wheel speed sensor signal value, thewheel deceleration derived from the sensor signal containing such errorswill naturally be inaccurate. Thus the wheel deceleration willoccasionally be perceived to exceed the set threshold erroneously,unnecessarily activating the anti-lock system and thus the electricmotor.

Such spurious operation of the anti-lock system could dangerously extendthe braking distance of the vehicle. In addition, the vehicle battery isloaded wastefully by the electric motor driving the fluid pump. Thus,repeated mis-activation of the anti-lock system could prove to be asafety problem and/or a serious drain on the battery.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide ananti-skid brake control system for an automotive vehicle which includesa power servo system operative during action of the anti-skid controlsystem for controlling braking pressure and inoperative while a wheelspeed is lower than a predetermined threshold while the anti-skidcontrol system is inoperative.

Another and more specific object of the invention is to provide ananti-skid brake control system in which a derived wheel acceleration anddeceleration will be modified to a given fixed value smaller than agiven pressure releasing threshold when a wheel speed is lower than thepredetermined threshold.

A further object of the invention is to provide an anti-skid brakecontrol system which can effectively and satisfactorily avoids pressurereleasing operation when the wheel speed is lower than the predeterminedthreshold, whereby avoiding waisting of a vehicle battery power.

In order to accomplish the above-mentioned and other objects, ananti-skid brake control system, according to the present invention,includes a detector which detects a wheel speed lower than apredetermined wheel speed threshold. The detector produces a detectorsignal when the wheel speed is lower than the wheel speed threshold. Thesystem further includes another detector for deriving a wheelacceleration indicative signal having a value representative of a wheelacceleration and deceleration detected. The wheel accelerationindicative signal value is modified to a predetermined fixed value whichis smaller than a given deceleration threshold at which anti-skidcontrol is triggered, in response to the detector signal. Therefore, inaccordance with the present invention, anti-skid control is disabledwhile the wheel speed remains below the wheel speed threshold.

According to one aspect of the invention, an anti-skid brake controlsystem comprises a hydraulic brake circuit including a wheel cylinderfor applying braking pressure to a vehicle wheel, a pressure controlvalve disposed within the hydraulic brake circuit for increasing thefluid pressure in the wheel cylinder in its first position, decreasingthe fluid pressure in its second position and holding the fluid pressureconstant value in its third position, first means for detecting wheelspeed and producing a first signal having a value representative of thewheel speed, a second means for detecting wheel acceleration anddeceleration and producing a second signal having a value representativeof the wheel acceleration and deceleration, a third means, associatedwith the first and second means, for deriving control signal forselecting one of the first, second and third position of the pressurecontrol valve based on the first and second signals, the third meansproducing the control signal when the second signal value becomesgreater than a given deceleration threshold, and a fourth means,receiving the first signal, for producing a command to modify the secondsignal value to a predetermined value which is lower than thedeceleration threshold, when the first signal value becomes less than agiven wheel speed threshold.

According to another aspect of the invention, a method for controlling ahydraulic automotive brake system comprises the steps of detecting wheelspeed and producing a first signal indicative of the detected wheelspeed, detecting wheel acceleration and deceleration and producing asecond signal indicative of the detected wheel acceleration anddeceleration, controlling fluid pressure applied to a wheel cylinder ina hydraulic brake circuit by increasing, decreasing and maintaining thefluid pressure therein in accordance with the first and second signalvalues when wheel deceleration indicated by the second signal becomesgreater than a given deceleration threshold, and detecting when thefirst signal value is lower than a given wheel speed threshold and atthat time producing a signal indicative of a predetermined fixed value,and outputting the fixed value as a replacement for the second signal,which fixed value is less than the deceleration threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given below and from the accompanying drawings of thepreferred embodiments of the present invention, which, however, shouldnot be taken to limit the invention to the specific embodiments, but arefor explanation and understanding only.

FIG. 1 is a schematic block diagram of the general design of thepreferred embodiment of an anti-skid brake control system according tothe present invention;

FIG. 2 is a perspective illustration of the hydraulic circuits of theanti-skid brake system according to the present invention;

FIG. 3 is a circuit diagram of the hydraulic circuits performing theanti-skid control according to the present invention;

FIG. 4 is an illustration of the operation of an electromagnetic flowcontrol valve employed in the hydraulic circuit, which valve has beenshown in an application mode for increasing the fluid pressure in awheel cylinder;

FIG. 5 is a view similar to FIG. 4 but of the valve in a hold mode inwhich the fluid pressure in the wheel cylinder is held at asubstantially constant value;

FIG. 6 is a view similar to FIG. 4 but of the valve in a release mode inwhich the fluid pressure in the wheel cylinder is reduced;

FIG. 7 is a perspective view of a wheel speed sensor adapted to detectthe speed of a front wheel;

FIG. 8 is a side elevation of a wheel speed sensor adapted to detect thespeed of a rear wheel;

FIG. 9 is an explanatory illustration of the wheel speed sensors ofFIGS. 7 and 8;

FIG. 10 shows the waveform of an alternating current sensor signalproduced by the wheel speed sensor;

FIG. 11 is a timing chart for the anti-skid control system;

FIG. 12 is a block diagram of the preferred embodiment of a controllerunit in the anti-skid brake control system according to the presentinvention;

FIG. 13 is a flowchart of a main program of a microcomputer constitutingthe controller unit of FIG. 12;

FIG. 14 is a flowchart of an interrupt program executed by thecontroller unit;

FIG. 15 is a flowchart of a main routine in the main program of FIG. 13;

FIG. 16 is a flowchart of an output calculation program for deriving EVand V signals for controlling operation mode of the electromagneticvalve according to the valve conditions of FIGS. 4, 5 and 6;

FIG. 17 is a table determining the operation mode of the actuator 16,which table is accessed in terms of the wheel acceleration anddeceleration and the slip rate;

FIG. 18 shows an output characteristics of the wheel speed sensor;

FIG. 19 shows variations of wheel speed and wheel acceleration anddeceleration in a low speed range;

FIG. 20 is a flowchart of a wheel speed deriving routine in the outputcalculation program of FIG. 16;

FIG. 21 is a flowchart of a wheel acceleration and deceleration derivingroutine in the output calculation program of FIG. 16;

FIG. 22 is a similar graph to FIG. 19 but showing variatins of wheelspeed and wheel acceleration and deceleration as derived by theflowchart of FIGS. 20 and 21;

FIG. 23 is a flowchart of a modified wheel acceleration and decelerationderiving routine;

FIG. 24 shows similar graph to FIG. 22 but showing variations of wheelspeed and wheel acceleration and deceleration as derived by theflowchart of FIGS. 20 and 23;

FIG. 25 is a block diagram of another embodiment of the controller unitin the preferred embodiment of the anti-skid brake control systemaccording to the present invention;

FIG. 26 is a block diagram of the wheel acceleration and decelerationcalculation circuit in the controller unit of FIG. 25; and

FIG. 27 is a block diagram of the wheel acceleration and decelerationcalculation circuit as modified from that of FIG. 26.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This dapplication is one of eighteen mutually related co-pending PatentApplications in the United States, filed on the same day. All of theeighteen applications have been filed by the common applicant to thisapplication and commonly assigned to the assignee of this application.The other seventeen applications are identified below:

    ______________________________________                                                   U.S.                                                               Basic Japanese                                                                           Serial                                                             Patent Appln. No.                                                                        No.     Title of the Invention                                     ______________________________________                                        Showa 58-70891                                                                           601,326 AN AUTOMOTIVE                                                                 ANTI-SKID BRAKE                                                               CONTROL SYSTEM WITH                                                           SAMPLING INPUT TIME                                                           DATA OF WHEEL SPEED                                                           SENSOR SIGNALS                                             Showa 58-70892                                                                           601,375 METHOD AND SYSTEM FOR                                      (filed April 23,   SAMPLING INPUT TIME                                        1983)              DATA FOR WHEEL SPEED                                                          SENSOR IN AN                                                                  AUTOMOTIVE ANTI-SKID                                                          BRAKE CONTROL SYSTEM                                       Showa 58-70893                                                                           601,325 AUTOMOTIVE ANTI-SKID                                       (filed April 23,   CONTROL SYSTEM WITH                                        1983)              CONTROL OF SAMPLING OF                                                        INPUT TIME DATA OF                                                            WHEEL SPEED SENSOR                                                            SIGNALES AND METHOD                                                           THEREFOR                                                   Showa 58-70894                                                                           601,317 ANTI-SKID CONTROL                                          (filed April 23,   SYSTEM FOR AUTOMOTIVE                                      1983)              BRAKE SYSTEM WITH                                                             SAMPLE CONTROL FOR                                                            SAMPLING INPUT TIMING                                                         OF SENSOR SIGNAL                                                              PULSES WITH REQUIRED                                                          PROCESS IDENTIFICATION                                                        AND METHOD FOR                                                                SAMPLING                                                   Showa 58-70895                                                                           601,294 ANTI-SKID BRAKE                                            (filed April 23,   CONTROL SYSTEM                                             1983)              INCLUDING A PROCEDURE                                                         OF SAMPLING OF INPUT                                                          TIME DATA OF WHEEL                                                            SPEED SENSOR SIGNALS                                                          AND METHOD THEREFOR                                        Showa 58-70896                                                                           601,344 ANTI-SKID BRAKE                                            (filed April 23,   CONTROL SYSTEM                                             1983)              INCLUDING WHEEL                                                               DECELERATION                                                                  CALCULATION WITH                                                              SHORTER LAG-TIME AND                                                          METHOD FOR PERFORMING                                                         CALCULATION                                                Showa 58-70897                                                                           601,338 ANTI-SKID BRAKE                                            (filed April 23,   CONTROL SYSTEM WITH                                        1983)              SAMPLE CONTROL OF                                                             SENSOR SIGNAL INPUT                                                           TIME DATA, AND METHOD                                                         THEREFOR                                                   Showa 58-70898                                                                           601,337 ANTI-SKID BRAKE                                            (filed April 23,   CONTROL SYSTEM WITH                                        1983)              CONTROL OF SAMPLING                                                           TIMING OF INPUT TIMING                                                        VALUES OF WHEEL SPEED                                                         SENSOR SIGNAL PULSES                                       Showa 58-70899                                                                           601,330 ANTI-SKID BRAKE                                            (filed April 23,   CONTROL SYSTEM FOR                                         1983)              AUTOMOTIVE VEHICLE                                         Showa 58-70900                                                                           601,364 ANTI-SKID BRAKE                                            (filed April 23,   CONTROL SYSTEM WITH                                        1983)              REDUCED DURATION OF                                                           WHEEL ACCELERATION AND                                                        DECELERATION                                                                  CALCULATION                                                Showa 58-84088                                                                           601,363 ANTI-SKID BRAKE                                            (filed May 16,     CONTROL SYSTEM WITH                                        1983)              OPERATIONAL MODE                                                              CONTROL AND METHOD                                                            THEREFOR                                                   Showa 58-84082                                                                           601,318 METHOD AND SYSTEM FOR                                      (filed May 16,     DERIVING WHEEL                                             1983)              ROTATION SPEED DATA                                                           FOR AUTOMOTIVE                                                                ANTI-SKID CONTROL                                          Showa 58-84085                                                                           601,345 METHOD AND SYSTEM FOR                                      (filed May 16,     DERIVING WHEEL                                             1983)              ACCELERATION AND                                                              DECELERATION IN                                                               AUTOMOTIVE ANTI-SKID                                                          BRAKE CONTROL SYSTEM                                       Showa 58-84092                                                                           601,293 ANTI-SKID BRAKE                                            (filed May, 16     CONTROL SYSTEM AND                                         1983)              METHOD FEATURING                                                              VEHICLE BATTERY                                                               PROTECTION                                                 Showa 58-84081                                                                           601,327 METHOD AND SYSTEM FOR                                      (filed May, 16     DERIVING WHEEL                                             1983)              ROTATION SPEED DATA                                                           FOR AUTOMOTIVE                                                                ANTI-SKID CONTROL                                          Showa 58-84090                                                                           601,258 ANTI-SKID BRAKE                                            (filed May, 16     CONTROL SYSTEM                                             1983)              INCLUDING FLUID PUMP                                                          AND DRIVE CIRCUIT                                                             THEREFOR                                                   Showa 58-102919                                                                          601,295 ANTI-SKID BRAKE                                            & 58-109308        CONTROL SYSTEM WITH A                                      (respectively      PLURALITY OF                                               filed              INDEPENDENTLY                                              June 10, 1983 &    OPERATIVE DIGITAL                                          June 20, 1983)     CONTROLLERS                                                ______________________________________                                    

Disclosures of other seventeen applications as identified above arehereby incorporated by reference for the sake of disclosure.

Referring now to the drawings, particularly to FIG. 1, the preferredembodiment of an anti-skid control system according to the presentinvention includes a control module 200 including a front-leftcontroller unit (FL) 202, a front-right controller unit (FR) 204 and arear controller unit (R) 206. The controller unit 202 comprises amicroprocessor and is adapted to control brake pressure applied to afront left wheel cylinder 30a of a front left hydraulic brake system 302of an automotive hydraulic brake system 300. Similarly, the controllerunit 204 is adapted to control brake pressure applied to the wheelcylinder 34a of a front right wheel (not shown) in the front righthydraulic brake system 304 and the controller unit 206 is adapted tocontrol brake pressure applied to the rear wheel cylinders 38a of thehydraulic rear brake system 306. Respective brake systems 302, 304 and306 have electromagnetically operated actuators 16, 18 and 20, each ofwhich controls the pressure of working fluid in the corresponding wheelcylinders. By means of the controlled pressure, the wheel cylinders 30a,34a and 38a apply braking force to brake disc rotors 28, 32 and 36mounted on the corresponding wheel axles for rotation with thecorresponding vehicle wheels via brake shoe assemblies 30, 34 and 38.

Though the shown brake system comprises disc brakes, the anti-skidcontrol system according to the present invention can also be applied todrum-type brake systems.

The controller units 202, 204 and 206 are respectively associated withactuator drive circuits 214, 216 and 218 to control operations ofcorresponding actuators 16, 18 and 20. In addition, each of thecontroller units 202, 204 and 206 is connected to a corresponding wheelspeed sensor 10, 12 and 14 via shaping circuits 208, 210 and 212incorporated in the controller 200. Each of the wheel speed sensors 10,12 and 14 is adapted to produce an alternating-current sensor signalhaving a frequency related to or proportional to the rotation speed ofthe corresponding vehicle wheel. Each of the A-C sensor signals isconverted by the corresponding shaping circuit 208, 210 and 212 into arectangular pulse signal which will be hereafter referred to as "sensorpulse signal". As can be appreciated, since the frequency of the A-Csensor signals is proportional to the wheel speed, the frequency of thesensor pulse signal should correspond to the wheel rotation speed andthe pulse intervals thereof will be inversely proportional to the wheelrotation speed.

The controller units 202, 204 and 206 operate independently andcontinuously process the sensor pulse signal to derive control signalsfor controlling the fluid pressure in each of the wheel cylinders 30a,34a and 38a in such a way that the slip rate R at each of the vehiclewheels is optimized to shorten the distance required to stop thevehicle, which distance will be hereafter referred to as "brakingdistance".

In general, each controller unit 202, 204 and 206 monitors receipt ofthe corresponding sensor pulses so that it can derive the pulse intervalbetween the times of receipt of successive sensor pulses. Based on thederived pulse interval, the controller units 202, 204 and 206 calculateinstantaneous wheel speed V_(w) and instantaneous wheel acceleration ordeceleration a_(w). From these measured and derived values, a targetwheel speed V_(i) is derived, which is an assumed value derived from thewheel speed at which a slip is assumed to be zero or approximately zero.The target wheel speed V_(i) varies at a constant decelerating ratederived from variation of the wheel speed. The target wheel speed thuscorresponds to a vehicle speed which itself is based on variations ofthe wheel speed. Based on the difference between the instantaneous wheelspeed V_(w) and the target wheel speed V_(i), a slip rate R is derived.The controller units 202, 204 and 206 determine the appropriateoperational mode for increasing, decreasing or holding the hydraulicbrake pressure applied to the wheel cylinders 30a, 34a and 38 a. Thecontrol mode in which the brake pressure is increased will be hereafterreferred to as "application mode". The control mode in which the brakepressure is decreased will be hereafter referred to as "release mode".The mode in which the brake pressure is held essentially constant willbe hereafter referred to as "hold mode". The anti-skid control operationconsists of a loop of application mode, hold mode, release mode and holdmode. This loop is repeated throughout the anti-skid brake controloperation cyclically. One cycle of the loop of the the control variationwill be hereafter referred to as "skid cycle".

FIG. 2 shows portions of the hydraulic brake system of an automotivevehicle to which the preferred embodiment of the anti-skid controlsystem is applied. The wheel speed sensors 10 and 12 are respectivelyprovided adjacent the brake disc rotor 28 and 32 for rotation therewithso as to produce sensor signals having frequencies proportional to thewheel rotation speed and variable in accordance with variation of thewheel speed. On the other hand, the wheel speed sensor 14 is providedadjacent a propeller shaft near the differential gear box or drivepinion shaft 116 for rotation therewith. (See FIG. 8) Since the rotationspeeds of the left and right rear wheels are free to vary independentlydepending upon driving conditions due to the effect of the differentialgear box 40, the rear wheel speed detected by the rear wheel speedsensor 14 is the average of the speeds of the left and right wheels.Throughout the specification, "rear wheel speed" will mean the averagerotation speed of the left and right rear wheels.

As shown in FIG. 2, the actuator unit 300 is connected to a master wheelcylinder 24 via primary and secondary outlet ports 41 and 43 thereof andvia pressure lines 44 and 42. The master wheel cylinder 24 is, in turn,associated with a brake pedal 22 via a power booster 26 which is adaptedto boost the braking force applied to the brake pedal 22 before applyingsame to the master cylinder. The actuator unit 300 is also connected towheel cylinders 30a, 34a and 38a via brake pressure lines 46, 48 and 50.

The circuit lay-out of the hydraulic brake system circuit will bedescribed in detail below with reference to FIG. 3 which is only anexample of the hydraulic brake system to which the preferred embodimentof the anti-skid control system according to the present invention canbe applied, and so it should be appreciated that it is not intended tolimit the hydraulic system to the embodiment shown. In FIG. 3, thesecondary outlet port 43 is connected to the inlet ports 16b and 18b ofelectromagnetic flow control valves 16a and 18a, the respective outletports 16c and 18c of which are connected to corresponding left and rightwheel cylinders 30a and 34a, via the secondary pressure lines 46 and 48.The primary outlet port 41 is connected to the inlet port 20b of theelectromagnetic valve 20a, the outlet port 20c of which is connected tothe rear wheel cylinders 38a, via a primary pressure line 50. Theelectromagnetic valves 16a, 18a and 20a also have drain ports 16d, 18dand 20d. The drain ports 16d and 18d are connected to the inlet port 72aof a fluid pump 90 via drain passages 80, 82 and 78. The fluid pump 90is associated with an electric motor 88 to be driven by the latter whichis, in turn, connected to a motor relay 92, the duty cycle of which iscontrolled by means of a control signal from the control module 200.While the motor relay 92 is energized to be turned ON, the motor 88 isin operation to drive the fluid pump 90. The drain port 20d of theelectromagnetic flow control valve 20a is connected to the inlet port58a of the fluid pump 90 via a drain passage 64.

The outlet ports 72b and 58b are respectively connected to the pressurelines 42 and 44 via a return passages 72c and 58c. The outlet ports 16c,18c and 20c of respective electromagnetic flow control valves 16a, 18aand 20a are connected to corresponding wheel cylinders 30a, 34a and 38avia braking lines 46, 48 and 50. Bypass passages 96 and 98 are providedto connect the braking pressure lines 46 and 48, and 50 respectively tothe pressure lines 42 and 44, bypassing the electromagnetic flow controlvalves.

Pump pressure check valves 52 and 66 are installed in the pressure lines42 and 44. Each of the pump pressure check valves 66 and 52 is adaptedto prevent the working fluid pressurized by the fluid pump 90 fromtransmitting pressure surges to the master cylinder 24. Since the fluidpump 90 is designed for quick release of the braking pressure in thebraking pressure lines 46, 48 and 50 and thus releasing the wheelcylinders 30a, 34a and 38a from the braking pressure, it is active uponrelease mode of the anti-skid control. The fluid pump 90 remainsoperation throughout the duration in which anti-skid brake control iscontinued. This would result in pressure surges in the working fluidfrom the fluid pump 90 to the master cylinder 24 if the pump pressurecheck valves 66 and 52 were not provided. The pump pressure check valves66 and 52 serve as one-way check valves allowing fluid flow from themaster cylinder 24 to the inlet ports 16b, 18b and 20b of theelectromagnetic valves 16a, 18a and 20a. Pressure accumulators 70 and 56are installed in the pressure lines 42 and 44, which pressureaccumulators serve to accumulate fluid pressure generated at the outletports 72b and 58b of the fluid pump 90 while the inlet ports 16b, 18band 20b are closed. Toward this end, the pressure accumulators 70 and 56are connected to the outlet ports 72b and 58b of the fluid pump 90 viathe return passages 72c and 58c. Outlet valves 68 and 54 are one-waycheck valves allowing one-way fluid communication from the fluid pump tothe pressure accumulators. These outlet valves 68 and 54 are effectivefor preventing the pressure accumulated in the pressure accumulators 70and 56 from surging to the fluid pump when the pump is deactivated. Inaddition, the outlet valves 68 and 54 are also effective to prevent thepressurized fluid flowing through the pressure lines 42 and 44 fromflowing into the fluid pump 90 through the return passages 72c and 58c.

Inlet check valves 74 and 60 are inserted in the drain passages 78 and64 for preventing surge flow of the pressurized fluid in the fluid pump90 to the electromagnetic flow control valves 16a, 18a and 20a after thebraking pressure in the wheel cylinders is released. The fluid flowingthrough the drain passages 78 and 64 is temporarily retained in fluidreservoirs 76 and 62 connected to the former.

Bypass check valves 86, 85 and 84 are inserted in the bypass passages 98and 96 for preventing the fluid in the pressure lines 42 and 44 fromflowing to the braking pressure lines 46, 48 and 50 without firstpassing through the electromagnetic flow control valves 16a, 18a and20a. On the other hand, the bypass check valves 86, 85 and 84 areadapted to permit fluid flow from the braking pressure line 46, 48 and50 to the pressure lines 42 and 44 when the master cylinder 24 isreleased and thus the line pressure in the pressure lines 42 and 44becomes lower than the pressure in the braking pressure lines 46, 48 and50.

The electromagnetic flow control valves 16a, 18a and 20a arerespectively associated with the actuators 16, 18 and 20 to becontrolled by means of the control signals from the control module 200.The actuators 16, 18 and 20 are all connected to the control module 200via an actuator relay 94, which thus controls the energization anddeenergization of them all. Operation of the electromagnetic valve 16ain cooperation with the actuator 16 will be illustrated with referenceto FIGS. 4, 5 and 6, in particular illustrating the application mode,hold mode and release mode, respectively.

It should be appreciated that the operation of the electromagneticvalves 18a and 20a are substantially the same as that of the valve 16a.Therefore, disclosure of the valve operations of the electromagneticvalves 18a and 20a is omitted in order to avoid unnecessary repetitionand for simplification of the disclosure.

APPLICATION MODE

In this position, the actuator 16 remains deenergized. An anchor of theelectromagnetic valve 16a thus remains in its initial position allowingfluid flow between the inert port 16b and the outlet port 16c so thatthe pressurized fluid supplied from the master cylinder 24 via thepressure line 42 may flow to the left front wheel cylinder 30a via thebraking pressure line 46. In this valve position, the drain port 16d isclosed to block fluid flow from the pressure line 42 to the drainpassage 78. As a result, the line pressure in the braking pressure line46 is increased in proportion to the magnitude of depression of thebrake pedal 22 and thereby the fluid pressure in the left front wheelcylinder 30a is increased correspondingly.

In this case, when the braking force applied to the brake pedal isreleased, the line pressure in the pressure line 42 drops due to returnof the master cylinder 24 to its initial position. As a result, the linepressure in the braking pressure line 46 becomes higher than that in thepressure line 42 and so opens the bypass valve 85 to permit fluid flowthrough the bypass passage 98 to return the working fluid to the fluidreservoir 24a of the master cylinder 24.

In the preferring construction, the pump pressure check valve 66,normally serving as a one-way check valve for preventing fluid flow fromthe electromagnetic valve 16a to the master cylinder 24, becomeswide-open in response to drop of the line pressure in the pressure linebelow a given pressure. This allows the fluid in the braking pressureline 46 to flow backwards through the electromagnetic valve 16a and thepump pressure check valve 66 to the master cylinder 24 via the pressureline 42. This function of the pump pressure check valve 66 facilitatesfull release of the braking pressure in the wheel cylinder 30a.

For instance, the bypass valve 85 is rated at a given set pressure, e.g.2 kg/cm² and closes when the pressure difference between the pressureline 42 and the braking pressure line 46 drops below the set pressure.As a result, fluid pressure approximating the bypass valve set pressuretends to remain in the braking pressure line 46, preventing the wheelcylinder 30a from returning to the fully released position. In order toavoid this, in the shown embodiment, the one-way check valve function ofthe pump pressure check valve 66 is disabled when the line pressure inthe pressure line 42 drops below a predetermined pressure, e.g. 10kg/cm². When the line pressure in the pressure line 42 drops below thepredetermined pressure, a bias force normally applied to the pumppressure check valve 66 is released, freeing the valve to allow fluidflow from the braking pressure line 46 to the master cylinder 24 via thepressure line 42.

HOLD MODE

In this control mode, a limited first value, e.g. 2 A of electriccurrent serving as the control signal is applied to the actuator 16 toposition the anchor closer to the actuator 16 than in the previous case.As a result, the inlet port 16b and the drain port 16d are closed toblock fluid communication between the pressure line 42 and the brakingpressure line 46 and between the braking pressure line and the drainpassage 78. Therefore, the fluid pressure in the braking pressure line46 is held at the level extant at the moment the actuator is operated bythe control signal.

In this case, the fluid pressure applied through the master cylinderflows through the pressure check valve 66 to the pressure accumulator70.

RELEASE MODE

In this control mode, a maximum value, e.g. 5 A of electric currentserving as the control signal is applied to the actuator 16 to shift theanchor all the way toward the actuator 16. As a result, the drain port16d is opened to establish fluid communication between the drain port16d and the outlet port 16c. At this time, the fluid pump 90 serves tofacilitate fluid flow from the braking pressure line 46 to the drainpassage 78. The fluid flowing through the drain passage is partlyaccumulated in the fluid reservoir 76 and the remainder flows to thepressure accumulator 70 via the check valves 60 and 54 and the fluidpump 90.

It will be appreciated that, even in this release mode, the fluidpressure in the pressure line 42 remains at a level higher or equal tothat in the braking pressure line 46, so that fluid flow from thebraking pressure line 46 to the pressure line 42 via the bypass passage98 and via the bypass check valve 85 will never occur.

In order to resume the braking pressure in the wheel cylinder (FL) 30aafter once the braking pressure is reduced by positioning theelectromagnetic valve 16a in the release position, the actuator 16 isagain deenergized. The electromagnetic valve 16a is thus returns to itsinitial position to allow the fluid flow between the inlet port 16b andthe outlet port 16c so that the pressurized fluid may flow to the leftfront wheel cylinder 30a via the braking pressure line 46. As set forththe drain port 16d is closed to block fluid flow from the pressure line42 to the drain passage 78.

As a result, the pressure accumulator 70 is connected to the left frontwheel cylinder 30a via the electromagnetic valve 16a and the brakingpressure line 46. The pressurized fluid in the pressure accumulator 70is thus supplied to the wheel cylinder 30a so as to resume the fluidpressure in the wheel cylinder 30a.

At this time, as the pressure accumulator 70 is connected to the fluidreservoir 76 via the check valves 60 and 54 which allow fluid flow fromthe fluid reservoir to the pressure accumulator, the extra amount ofpressurized fluid may be supplied from the fluid reservoir.

The design of the wheel speed sensors 10, 12 and 14 employed in thepreferred embodiment of the anti-skid control system will be describedin detail with reference to FIGS. 7 to 9.

FIG. 7 shows the structure of the wheel speed sensor 10 for detectingthe rate of rotation of the left front wheel. The wheel speed sensor 10generally comprises a sensor rotor 104 adapted to rotate with thevehicle wheel, and a sensor assembly 102 fixedly secured to the shimportion 106 of the knuckle spindle 108. The sensor rotor 104 is fixedlysecured to a wheel hub 109 for rotation with the vehicle wheel.

As shown in FIG. 9, the sensor rotor 104 is formed with a plurality ofsensor teeth 120 at regular angular intervals. The width of the teeth120 and the grooves 122 therebetween are equal in the shown embodimentand define a unit angle of wheel rotation. The sensor assembly 102comprises a magnetic core 124 aligned with its north pole (N) near thesensor rotor 104 and its south pole (S) distal from the sensor rotor. Ametal element 125 with a smaller diameter section 125a is attached tothe end of the magnetic core 124 nearer the sensor rotor. The free endof the mtal element 125 faces the sensor teeth 120. An electromagneticcoil 126 encircles the smaller diameter section 125a of the metalelement. The electromagnetic coil 126 is adapted to detect variations inthe magnetic field generated by the magnetic core 124 to produce analternating-current sensor signal as shown in FIG. 10. That is, themetal element and the magnetic core 124 form a kind of proximity switchwhich adjusts the magnitude of the magnetic field depending upon thedistance between the free end of the metal element 124 and the sensorrotor surface. Thus, the intensity of the magnetic field fluctuates inrelation to the passage of the sensor teeth 120 and accordingly inrelation to the angular velocity of the wheel.

It should be appreciated that the wheel speed sensor 12 for the rightfront wheel has the substantially the same structure as the set forthabove. Therefore, explanation of the structure of the right wheel speedsensor 12 will be omitted in order to avoid unnecessary repetition inthe disclosure and in order to simplify the description.

FIG. 8 shows the structure of the rear wheel speed sensor 14. As withthe aforementioned left front wheel speed sensor 10, the rear wheelspeed sensor 14 comprises a sensor rotor 112 and a sensor assembly 102.The sensor rotor 112 is associated with a companion flange 114 which is,in turn, rigidly secured to a drive shaft 116 for rotation therewith.Thus, the sensor rotor 112 rotates with the drive shaft 116. The sensorassembly 102 is fixed to a final drive housing or a differential gearbox (not shown).

Each of the sensor assemblies applied to the left and right front wheelspeed sensors and the rear wheel sensor is adapted to output analternating-current sensor signal having a frequency proportional to orcorresponding to the rotational speed of the corresponding vehiclewheel. The electromagnetic coil 126 of each of the sensor assemblies 102is connected to the control module 100 to supply the sensor signalsthereto.

As set forth above, the control module 200 comprises the controller unit(FL) 202, the controller unit (FR) 204 and the controller unit (R) 206,each of which comprises a microcomputer. Therefore, the wheel speedsensors 10, 12 and 14 are connected to corresponding controller units202, 204 and 206 and send their sensor signals thereto. Since thestructure and operation of each of the controller units is substantiallythe same as that of the others, the structure and operation of only thecontroller unit 202 for performing the anti-skid brake control for thefront left wheel cylinder will be explained in detail.

FIG. 11 is a timing chart of the anti-skid control performed by thecontroller unit 202. As set forth above, the alternating-current sensorsignal output from the wheel speed sensor 10 is converted into arectangular pulse train, i.e. as the sensor pulse signal mentionedabove. The controller unit 202 monitors occurrences of sensor pulses andmeasures the intervals between individual pulses or between the firstpulses of groups of relatively-high-frequency pulses. Pulses are sogrouped that the measured intervals will exceed a predetermined value,which value will be hereafter referred to as "pulse interval threshold".

The wheel rotation speed V_(w) is calculated in response to each sensorpulse. As is well known, the wheel speed is generally inverselyproportional to the intervals between the sensor pulses, andaccordingly, the wheel speed V_(w) is derived from the interval betweenthe last sensor pulse input time and the current sensor pulse inputtime. A target wheel speed is designated V_(i) and the resultant wheelspeed is designated V_(w). In addition, the slip rate is derived fromthe rate of change of the wheel speed and an projected speed V_(v) whichis estimated from the wheel speed at the moment the brakes are appliedbased on the assumption of a continuous, linear deceleration withoutslippage. In general, the target wheel speed V_(i) is derived from thewheel speed of the last skid cycle during which the wheel decelerationrate was equal to or less than a given value which will be hereafterreferred to as "deceleration threshold a_(ref) ", and the wheel speed ofthe current skid cycle, and by estimating the rate of change of thewheel speed between wheel speeds at which the rate of deceleration isequal to or less than the deceleration threshold. In practice, the firsttarget wheel speed V_(i) is derived based on the projected speed V_(v)which corresponds to a wheel speed at the initial stage of brakingoperation and at which wheel deceleration exceeds a predetermined value,e.g. -1.2 G, and a predetermined deceleration rate, for example 0.4 G.The subsequent target wheel speed V_(i) is derived based on theprojected speeds V_(v) in last two skid cycles. For instance, thedeceleration rate of the target wheel speed V_(i) is derived from adifference of the projected speeds V_(v) in the last two skid cycle anda period of time in which wheel speed varies from the first projectedspeed to the next projected speed. Based on the last projected speed andthe deceleration rate, the target wheel speed in the current skid cycleis derived.

The acceleration and deceleration of the wheel is derived based on inputtimes of the three successive sensor pulses. Since the interval of theadjacent sensor signal pulses corresponds to the wheel speed, and thewheel speed is a function of the reciprocal of the interval, bycomparing adjacent pulse-to-pulse intervals, a value corresponding tothe variation or difference of the wheel speed may be obtained. Theresultant may be divided by the period of time in order to obtain thewheel acceleration and deceleration at the unit time. Therefore, theacceleration or deceleration of the wheel is derived from the followingequation: ##EQU1## where A, B and C are the input times of the sensorpulses in the order given.

On the other hand, the slip rate R is a rate of difference of wheelspeed relative to the vehicle speed which is assumed as substantiallycorresponding to the target wheel speed. Therefore, in the shownembodiment, the target wheel speed V_(i) is taken as variable orparameter indicative of the assumed or projected vehicle speed. The sliprate R can be obtained by dividing a difference between the target wheelspeed V_(i) and the instantaneous wheel speed V_(w) by the target wheelspeed. Therefore, in addition, the slip rate R is derived by solving thefollowing equation: ##EQU2##

Finally, the controller unit 202 determines the control mode, i.e.,release mode, hold mode and application mode from the slip rate R andthe wheel acceleration or deceleration a_(w).

General operation of the controller unit 202 will be briefly explainedherebelow with reference to FIG. 11. Assuming the brake is applied at t₀and the wheel deceleration a_(w) exceeds the predetermined value, e.g.1.2G at a time t₁, the controller unit 202 starts to operate at a timet₁. The first sensor pulse input time (t₁) is held in the controllerunit 202. Upon receipt of the subsequent sensor pulse at a time t₂, thewheel speed V_(w) is calculated by deriving the current sensor pulseperiod (dt=t₂ -t₁). In response to the subsequently received sensorpulses at times t₃, t₄ . . . , the wheel speed values V_(w2), V_(w3) . .. are calculated.

On the other hand, at the time t₁, the instantaneous wheel speed istaken as the projected speed V_(v). Based on the projected speed V_(v)and the predetermined fixed value, e.g. 0.4G, the target wheel speedV_(i) decelerating at the predetermined deceleration rate 0.4G isderived.

In anti-skid brake control, the braking force applied to the wheelcylinder is to be so adjusted that the peripheral speed of the wheel,i.e. the wheel speed, during braking is held to a given ratio, e.g. 85%to 80% of the vehicle speed. Therefore, the slip rate R has to bemaintained below a given ratio, i.e., 15% to 10%. In the preferredembodiment, the control system controls the braking force so as tomaintain the slip rate at about 15%. Therefore, a reference valueR_(ref) to be compared with the slip rate R is determined at a value at85% of the projected speed V_(v). As will be appreciated, the referencevalue is thus indicative of a slip rate threshold, which will behereafter referred to "slip rate threshold R_(ref) " throughout thespecification and claims, and varies according to variation of thetarget wheel speed.

In practical brake control operation performed by the preferredembodiment of the anti-skid control system according to the presentinvention, the electric current applied to the actuator attains alimited value, e.g., 2 A to place the electromagnetic valve 30a in thehold mode as shown in FIG. 5 when the wheel speed remains inbetween thetarget wheel speed V_(i) and the slip rate threshold R_(ref). When theslip rate derived from the target wheel speed V_(i) and the wheel speedV_(w) becomes equal to or larger than the slip rate threshold R_(ref),then the supply current to the actuator 16 is increased to a maximumvalue, e.g. 5 A to place the electromagnetic valve in the release modeas shown in FIG. 6. By maintaining the release mode, the wheel speedV_(w) is recovered to the target wheel speed. When the wheel speed isthus recovered or resumed so that the slip rate R at that wheel speedbecomes equal to or less than the slip rate threshold R_(ref), then thesupply current to the actuator 16 is dropped to the limited value, e.g.2 A to return the electromagnetic valve 30a to the hold mode. By holdingthe reduced fluid pressure in the wheel cylinder, the wheel speed V_(w)is further resumed to the target wheel speed V_(i). When the wheel speedV_(w) is resumed equal to or higher than the target wheel speed V_(i),the supply current is further dropped to zero for placing theelectromagnetic valve in the application mode as shown in FIG. 4. Theelectromagnetic valve 30a is maintained in the application mode untilthe wheel speed is decelerated at a wheel speed at which the wheeldeceleration becomes equal to or slightly more than the decelerationthreshold R_(ref) -1.2G. At the same time, the projected speed V_(v) isagain derived with respect to the wheel speed at which the wheeldeceleration a_(w) becomes equal to or slightly larger than thedeceleration threshold a_(ref). From a difference of speed of the lastprojected speed and the instant projected speed and the period of timefrom a time obtaining the last projected speed to a time obtaining theinstant projected speed, a deceleration rate of the target wheel speedV_(i) is derived. Therefore, assuming the last projected speed isV_(v1), the instant projected speed is V_(v2), and the period of time isT_(v), the target wheel speed V_(i) can be obtained from the followingequation:

    V.sub.i =V.sub.v2 -(V.sub.v1 -V.sub.v2)/T.sub.v ×t.sub.e

where t_(e) is an elapsed time from the time at which the instantprojected speed V_(v2) is obtained.

Based on the input timing to t₁, t₂, t₃, t₄ . . . , deceleration ratea_(w) is derived from the foregoing equation (1). In addition, theprojected speed V_(v) is estimated as a function of the wheel speedV_(w) and rate of change thereof. Based on the instantaneous wheelspeeds V_(w1) at which the wheel deceleration is equal to or less thanthe deceleration threshold a_(ref) and the predetermined fixed value,e.g. 0.4G for the first skid cycle of control operation, the targetwheel speed V_(i) is calculated. According to equation (2), the sliprate R is calculated, using successive wheel speed values V_(w1),V_(w2), V_(w3) . . . as parameters. The derived slip rate R is comparedwith the slip rate threshold R_(ref). As the wheel speed V_(w) dropsbelow the projected speed V_(v) at the time t₁, the controller unit 202switches the control mode from the application mode to the hold mode.Assuming also that the slip rate R exceeds the slip rate threshold atthe time t₄, then the controller unit 202 switches the control mode tothe release mode to release the fluid pressure at the wheel cylinder.

Upon release of the brake pressure in the wheel cylinder, the wheelspeed V_(w) recovers, i.e. the slip rate R drops until it is smallerthan the slip rate threshold at time t₇. The controller unit 202 detectswhen the slip rate R is smaller than the slip rate threshold R_(ref) andswitches the control mode from release mode to the hold mode.

By maintaining the brake system in the hold mode in which reduced brakepressure is applied to the wheel cylinder, the wheel speed increasesuntil it reaches the projected speed as indicated by the intersection ofthe dashed line (V_(v)) and the solid trace in the graph of V_(w) inFIG. 11. When the wheel speed V_(w) becomes equal to the target wheelspeed V_(i) (at a time t₈), the controller unit 202 switches the controlmode from the hold mode to the application mode.

As can be appreciated from the foregoing description, the control modewill tend to cycle through the control modes in the order applicationmode, hold mode, release mode and hold mode, as exemplified in theperiod of time from t₁ to t₈. This cycle of variation of the controlmodes will be referred to hereafter as "skid cycle". Practicallyspeaking, there will of course be some hunting and other minordeviations from the standard skid cycle.

The projected speed V_(v), which is meant to represent ideal vehiclespeed behavior, at time t₁ can be obtained directly from the wheel speedV_(w) at that time since zero slip is assumed. At the same time, thedeceleration rate of the vehicle will be assumed to be a predeterminedfixed value or the appropriate one of a family thereof, in order toenable calculation of the target wheel speed for the first skid cycleoperation. Specifically, in the shown example, the projected speed V_(v)at the time t₁ will be derived from the wheel speed V_(w1) at that time.Using the predetermined deceleration rate, the projected speed will becalculated at each time the wheel deceleration a_(w) in the applicationmode reaches the deceleration threshold a_(ref).

At time t₉, the wheel deceleration a_(w) becomes equal to or slightlylarger than the deceleration threshold a_(ref), then the secondprojected speed V_(v2) is obtained at a value equal to the instantaneouswheel speed V_(w) at the time t₉. According to the abovementionedequation, the deceleration rate da can be obtained

    da=(V.sub.v1 -V.sub.v2)/(t.sub.9 -t.sub.1)

Based on the derived deceleration rate da, the target wheel speed V_(i)' for the second skid cycle of control operation is derived by:

    V.sub.i '=V.sub.v2 -da×t.sub.e

Based on the derived target wheel speed, the slip rate threshold R_(ref)for the second cycle of control operation is also derived. As will beappreciated from FIG. 11, the control mode will be varied during thesecond cycle of skid control operation, to hold mode at time t₉ at whichthe wheel deceleration reaches the deceleration threshold a_(ref) as setforth above, to release mode at time t₁₀ at which the slip rate Rreaches the slip rate threshold R_(ref), to hold mode at time t₁₁ atwhich the slip rate R is recovered to the slip rate threshold R_(ref),and to application mode at time t₁₂ at which the wheel speed V_(w)recovered or resumed to the target wheel speed V_(i) '. Further, itshould be appreciated that in the subsequent cycles of the skid controloperations, the control of the operational mode of the electromagneticvalve as set forth with respect to the second cycle of controloperation, will be repeated.

Relating the above control operations to the structure of FIGS. 3through 6, when application mode is used, no electrical current isapplied to the actuator of the electromagnetic value 16a so that theinlet port 16b communicates with the outlet port 16c, allowing fluidflow between the pressure passage 42 and the brake pressure line 46. Alimited amount of electrical current (e.g. 2A) is applied at times t₁,t₇, t₉ and t₁₁, so as to actuate the electromagnetic value 16a to itslimited stroke position by means of the actuator 16, and the maximumcurrent is applied to the actuator as long as the wheel speed V_(w) isnot less than the projected speed and the slip rate is greater than theslip rate threshold R_(ref). Therefore, in the shown example, thecontrol mode is switched from the application mode to the hold mode attime t₁ and then to the release mode at time t₄. At time t₇, the sliprate increases back up to the slip rate threshold R.sub. ref, so thatthe control mode returns to the hold mode, the actuator driving theelectromagnetic valve 16a to its central holding position with thelimited amount of electrical current as the control signal. When thewheel speed V_(w) finally returns to the level of the target wheel speedV_(i) at time t₈, the actuator 16 supply current is cut off so that theelectromagnetic valve 16a returns to its rest position in order toestablish fluid communication between the pressure line 42 and thebraking pressure line 46 via inlet and outlet ports 16b and 16c.

Referring to FIG. 12, the controller unit 202 includes an inputinterface 230, CPU 232, an output interface 234, RAM 236 and ROM 238.The input interface 230 includes an interrupt command generator 229which produces an interrupt command in response to every sensor pulse.In ROM, a plurality of programs including a main program (FIG. 13), aninterrupt program (FIG. 15), an sample control program (FIG. 19), atimer overflow program (FIG. 20) and an output calculation program (FIG.23) are stored in respectively corresponding address blocks 244, 246,250, 252 and 254.

The input interface also has a temporary register for temporarilyholding input timing for the sensor pulses. RAM 236 similarly has amemory block holding input timing for the sensor pulses. The contents ofthe memory block 240 of RAM may be shifted whenever calculations of thepulse interval, wheel speed, wheel acceleration or deceleration, targetwheel speed, slip rate and so forth are completed. One method ofshifting the contents is known from the corresponding disclosure of theU.S. Pat. No. 4,408,290. The disclosure of the U.S. Pat. No. 4,408,290is hereby incorporated by reference. RAM also has a memory block 242 forholding pulse intervals of the input sensor pulses. The memory block 242is also adapted to shift the contents thereof according to the mannersimilar to set forth in the U.S. Pat. No. 4,408,290.

An interrupt flag 256 is provided in the controller unit 202 forsignalling interrupt requests to the CPU. The interrupt flag 256 is setin response to the interrupt command from the interrupt commandgenerator 229. A timer overflow interrupt flag 258 is adapted to set anoverflow flag when the measured interval between any pair of monitoredsensor pulses exceeds the a capacity of a clock counter.

In order to time the arrival of the sensor pulses, a clock is connectedto the controller unit 202 to feed time signals indicative of elapsedreal time. The timer signal value is latched whenever a sensor pulse isreceived and stored in either or both of the temporary register 231 inthe input interface 230 and the memory block 240 of RAM 236.

The operation of the controller unit 202 and the function of eachelements mentioned above will be described with reference to FIGS. 13 to27.

FIG. 13 illustrates the main program for the anti-skid control system.Practically speaking, this program will generally be executed as abackground job, i.e. it will have a lower priority than most otherprograms under the control of the same processor. Its first step 1002 isto wait until at least one sample period, covering a single sensor pulseor a group thereof, as described in more detail below, is completed asindicated when a sample flag FL has a non-zero value. In subsequent step1004, the sample flag FL is checked for a value greater than one, whichwould indicate the sample period is too short. If this is the case,control passes to a sample control program labelled "1006" in FIG. 13but shown in more detail in FIG. 19. If FL=1, then the control processis according to plan, and control passes to a main routine explainedlater with reference to FIG. 15. Finally, after completion of the mainroutine, a time overflow flag OFL is reset to signify successfulcompletion of another sample processing cycle, and the main programends.

FIG. 14 shows the interrupt program stored in the memory block 246 ofROM 238 and executed in response to the interrupt command generated bythe interrupt command generator 229 whenever a sensor pulse is received.It should be noted that a counter value NC of an auxiliary counter 233is initially set to 1, a register N representing the frequency dividerratio is set at 1, and a counter value M of an auxiliary counter 235 isset at -1. After starting execution of the interrupt program, thecounter value NC of the auxiliary counter 233 is decremented by 1 atblock 3002. The auxiliary counter value NC is then checked at a block3004 for a value greater than zero. For the first sensor pulse, sincethe counter value NC is decremented by 1 (1-1=0) at the block 3002 andthus is zero, the answer of the block 3004 is NO. In this case, theclock counter value t is latched in a temporary register 231 in theinput interface 230 at a block 3006. The counter value NC of theauxiliary counter 233 is thereafter assigned the value N in a register235, which register value N is representative of frequency dividingratio determined during execution of the main routine explained later,at a block 3008. The value M of an auxiliary counter 235 is thenincremented by 1. The counter value M of the auxiliary counter 235labels each of a sequence of sample periods covering an increasingnumber of sensor pulses. After this, the sample flag FL is incrementedby 1 at a block 3012. After the block 3012, interrupt program ends,returning control to the main program or back to block 3002, whichevercomes first.

On the other hand, when the counter value NC is non-zero when checked atthe block 3004, this indicates that not all of the pulses for thissample period have been received, and so the interrupt program endsimmediately.

This interrupt routine thus serves to monitor the input timing t of eachpulse sampling period, i.e. the time t required to receive NC pulses,and signals completion of each sample period (M=0 through M=10, forexample) for the information of the main program.

Before describing the operation in the main routine, the general methodof grouping the sensor pulses into sample periods will be explained tofacilitate understanding of the description of the operation in the mainroutine.

In order to enable the controller unit 202 to accurately calculate thewheel acceleration and deceleration a_(w), it is necessary that thedifference between the pulse intervals of the single or grouped sensorpulses exceeding a given period of time, e.g. 4 ms. In order to obtainthe pulse interval difference exceeding the given period of time, 4 ms,which given period of time will be hereafter referred to as "pulseinterval threshold S", some sensor pulses are ignored so that therecorded input timing t of the sensor pulse groups can satisfy thefollowing formula:

    dT=(C-B)-(B-A)≧S(4 ms.)                             (3)

where A, B and C are the input times of three successive sensor pulsegroups.

The controller unit 202 has different sample modes, i.e. MODE 1, MODE 2,MODE 3 and MODE 4 determining the number of sensor pulses in each sampleperiod group. As shown in FIG. 16, in MODE 1 every sensor pulse inputtime is recorded and therefore the register value N is 1. In MODE 2,every other sensor pulse is ignored and the register value N is 2. InMODE 3, every fourth sensor pulse is monitored, i.e. its input time isrecorded, and the register value N is 4. In MODE 4, every eighth sensorpulse is sampled and the register value N is then 8.

The controller unit 202 thus samples the input timing of threesuccessive sensor pulses to calculate the pulse interval difference dTwhile operating in MODE 1. If the derived pulse interval difference isequal to or greater than the pulse interval threshold S, then sensorpulses will continue to be sampled in MODE 1. Otherwise, the inputtiming of every other sensor pulse is sampled in MODE 2 and from thesampled input timing of the next three sensor pulses sampled, the pulseinterval difference dT is calculated to again be compared with the pulseinterval threshold S. If the derived pulse interval difference is equalto or greater than the pulse interval threshold S, we remain in MODE 2.Otherwise, every four sensor pulses are sampled in MODE 3. The inputtimings of the next three sampled sensor pulses are processed to derivethe pulse interval difference dT. The derived pulse interval differencedT is again compared with the pulse interval threshold S. If the derivedpulse interval difference is equal to or greater than the pulse intervalthreshold S, the MODE remains at 3 and the value N is set to 4. On theother hand, if the derived pulse interval difference dT is less than thepulse interval threshold S, the mode is shifted to MODE 4 to sample theinput timing of every eighth sensor pulse. In this MODE 4, the value Nis set at 8.

Referring to FIG. 15, the main routine serves to periodically derive anupdated wheel acceleration rate value a_(w). In general, this is done bysampling larger and larger groups of pulses until the difference betweenthe durations of the groups is large enough to yield an accurate value.In the main routine, the sample flag FL is reset to zero at a block2001. Then the counter value M of the auxiliary counter 233, indicatingthe current sample period of the current a_(w) calculation cycle, isread out at a block 2002 to dictate the subsequent program steps.

Specifically, after the first sample period (M=φ), the input timing ttemporarily stored in the temporary register 231 corresponding to thesensor pulse number (M=0) is read out and transferred to a memory block240 of RAM at a block 2004, which memory block 240 will be hereafterreferred to as "input timing memory". Then control passes to the block1008 of the main program. When M=2, the corresponding input timing t isread out from the temporary register 231 and transferred to the inputtiming memory 240 at a block 2006. Then, at a block 2008, a pulseinterval Ts between the sensor pulses of M=1 is derived from the twoinput timing values in the input timing memory 240. That is, the pulseinterval of the sensor pulse (M=1) is derived by:

    Ts32 t.sub.1 -t.sub.0

where

t₁ is input time of the sensor pulse M1; and

t₀ is input time of the sensor pulse M0.

The derived pulse interval T_(s) of the sensor pulse M1 is then comparedwith a reference value, e.g. 4 ms., at a block 2010. If the pulseinterval T_(s) is shorter than the reference value, 4 ms., controlpasses to a block 2012 wherein the value N and the pulse interval T_(s)are multiplied by 2. The doubled timing value (2T_(s)) is again comparedwith the reference value by returning to the block 2010. The blocks 2010and 2012 constitute a loop which is repeated until the pulse interval(2nT_(s)) exceeds the reference value. When the pulse interval (2nT_(s))exceeds the reference value at the block 2010, a corresponding value ofN (2N) is automatically selected. This N value represents the number ofpulses to be treated as a single pulse with regard to timing.

After setting the value of N and thus deriving the sensor pulse groupsize, then the auxiliary counter value NC is set to 1, at a block 2016.The register value N is then checked for a value of 1, at a block 2018.If N=1, then the auxiliary counter value M is set to 3 at a block 2020and otherwise control returns to the main program. When the registervalue N equals 1, the next sensor pulse, which would normally beignored, will instead be treated as the sensor pulse having the sampleperiod number M=3.

In the processing path for the sample period number M=3, thecorresponding input timing is read from the corresponding address of thetemporary register 231 and transferred to the input timing memory 240,at a block 2024. The pulse interval T₂ between the sensor pulses at M=1and M=3 is then calculated at a block 2026. The derived pulse intervalT₂ is written in a storage section of a memory block 242 of RAM 236 fora current pulse interval data, which storage section will be hereafterreferred at as "first pulse interval storage" and which memory block 242will be hereafter referred to as "pulse interval memory". After theblock 2026, control returns to the main program to await the next sensorpulse, i.e. the sensor pulse for sample period number M=4.

When the sensor pulse for M=4 is received, the value t of the temporaryregister 231 is read out and transferred to the input timing memory 240at block 2028. Based on the input timing of the sensor pulses for M=3and M=4, the pulse interval T₃ is calculated at a block 2030. The pulseinterval T₃ derived at the block 2030 is then written into the firstpulse interval storage of the pulse interval memory. At the same time,the pulse interval data T₂ previously stored in the first pulse intervalstorage is transferred to another storage section of the pulse intervalmemory adapted to store previous pulse interval data. This other storagesection will be hereafter referred to as "second pulse intervalstorage". Subsequently, at a block 2032 the contents of the first andsecond storages, i.e. the pulse interval data T₂ and T₃ are read out.Based on the read out pulse interval data T₂ and T₃, a pulse intervaldifference dT is calculated at block 2032 and compared with the pulseinterval threshold S to determine whether or not the pulse intervaldifference dT is large enough for accurate calculation of wheelacceleration or deceleration a_(w). If so, process goes to the block2040 to calculate the wheel acceleration or deceleration according tothe equation (1). The register value N is then set to 1 at the block2044 and thus MODE 1 is selected. In addition sample period number M isreset to -1, and the a_(w) derivation cycle starts again. On the otherhand, if at the block 2032 the pulse interval difference dT is too smallto calculate the wheel acceleration or deceleration a_(w), then thevalue N is multiplied by 2 at a block 2034. Due the updating of thevalue N, the sample mode of the sensor pulses is shifted to the nextmode.

When the block 2034 is performed and thus the sample mode is shifted toMODE 2 with respect to the sensor pulse of M=4', the sensor pulse c₂input following to the sensor pulse of M=4' is ignored. The sensor pulsec₃ following to the ignored sensor pulse c₂ is then taken as the sensorpulse to be sampled as M=3". At this time, the sensor pulse of M=4' istreated as the sensor pulse of M=2" and the sensor pulse of M=2 istreated as the sensor pulse of M=1". Therefore, calculation of theinterval difference dT and discrimination if the derived intervaldifference dT is greater than the pulse interval threshold S in theblock 2032 will be carried out with respect to the sensor pulse c₃ whichwill be treated as the sensor pulse of M=4". The blocks 2032 and 2034are repeated until the interval difference greater than the pulseinterval threshold S is obtained. The procedure taken in each cycle ofrepetition of the blocks 2032 and 2034 is substantially same as that setforth above.

As set forth above, by setting the counter value NC of the auxiliarycounter 233 to 1 at the block 2016, the input timing of the sensor pulsereceived immediately after initially deriving the sample mode at theblocks 2010 and 2012 will be sampled as the first input timing to beused for calculation of the wheel acceleration and deceleration. Thismay be contrasted with the procedure taken in the known art.

FIG. 16 shows the output program for deriving the wheel speed V_(w),wheel acceleration and deceleration a_(w) and slip rate R, selecting theoperational mode, i.e. application mode, hold mode and release mode andoutputting an inlet signal EV and/or an outlet signal AV depending uponthe selected operation mode of the actuator 16.

When the application mode is selected the inlet signal EV goes HIGH andthe outlet signal EV goes HIGH. When the release mode is selected, theinlet signal EV goes LOW and the outlet signal AV also goes LOW. Whenthe selected mode is the hold mode, the inlet signal EV remains HIGHwhile the outlet signal AV goes LOW. These combinations of the inletsignal EV and the outlet signal AV correspond to the actuator supplycurrent levels shown in FIG. 11 and thus actuate the electromagneticvalve to the corresponding positions illustrated in FIGS. 4, 5 and 6.

The output program is stored in the memory block 254 and adapted to beread out periodically, e.g. every 10 ms, to be executed as an interruptprogram.

During execution of the output calculation program, the pulse interval Tis read out from a memory block 241 of RAM which stores the pulseinterval, at a block 5002. As set forth above, since the pulse intervalT is inversely proportional to the wheel rotation speed V_(w), the wheelspeed can be derived by calculating the reciprocal (1/T) of the pulseinterval T. This calculation of the wheel speed V_(w) is performed at ablock 5004 in the output program. After the block 5004, the target wheelspeed V_(i) is calculated at a block 5006. The manner of deriving thetarget wheel speed V_(i) has been illustrated in the U.S. Pat. Nos.4,392,202 to Toshiro MATSUDA, issued on July 5, 1983, 4,384,330 also toToshiro MATSUDA, issued May 17, 1983 and 4,430,714 also to ToshiroMATSUDA, issued on Feb. 7, 1984, which are also assigned to the assigneeof the present invention. The disclosure of the above-identified threeUnited States Patents are hereby incorporated by reference for the sakeof disclosure. As is obvious herefrom, the target wheel speed V_(i) isderived as a function of wheel speed deceleration as actually detected.For instance, the wheel speed V_(w) at which the wheel decelerationa_(w) exceeds the deceleration threshold a_(ref) is taken as onereference point for deriving the target wheel speed V_(i). The wheelspeed at which the wheel deceleration a_(w) also exceeds thedeceleration threshold a_(ref) is taken as the other reference point. Inaddition, the period of time between the points a and b is measured.Based on the wheel speed V_(w1) and V_(w2) and the measured period P,the deceleration rate dV_(i) is derived from:

    dV.sub.i =(V.sub.w1 -V.sub.w2)/P                           (4)

This target wheel speed V_(i) is used for skid control in the next skidcycle.

It should be appreciated that in the first skid cycle, the target wheelspeed V_(i) cannot be obtained. Therefore, for the first skid cycle, apredetermined fixed value will be used as the target wheel speed V_(i).

At a block 5008 in FIG. 16, the slip rate R is calculated according tothe foregoing formula (2). Subsequently the operational mode isdetermined on the basis of the wheel acceleration and deceleration a_(w)and the slip rate R, at a block 5010. FIG. 17 shows a table used indetermining or selecting the operational mode of the actuator 16 andwhich is accessed according to the wheel acceleration and decelerationa_(w) and the slip rate R. As can be seen, when the wheel slip rate R isin the range of 0 to 15%, the hold mode is selected when the wheelacceleration and deceleration a_(w) is lower than -1.0G and theapplication mode is selected when the wheel acceleration anddeceleration a_(w) is in the range of -1.0G to 0.6 G. On the other hand,when the slip rate R remains above 15%, the release mode is selectedwhen the wheel acceleration and deceleration a_(w) is equal to or lessthan 0.6G, and the hold mode is selected when the wheel acceleration anddeceleration is in a range of 0.6 G to 1.5G. When the wheel accelerationand deceleration a_(w) is equal to or greater than 1.5G, the applicationmode is selected regardless of the slip rate.

Further, as will be seen from FIG. 18, the wheel sensor output level isvariable depending upon the wheel speed to be detected. In substantiallow and high wheel speed range, the output level of the wheel speedsensor may drop to a lower level in relation to a given thresholdV_(th). In the wheel speed range in the hatched area of FIG. 18, theoutput level of the wheel speed sensor becomes lower than the giventhreshold V_(th) due to substantially low and high wheel speed. In thisrange, the fluctuation of sensor output reaches a significant level andmay cause error. For instance, as shown in FIG. 19, when the wheel speeddrops lower than the given threshold level at time p₁, the sensor signalpulses indicative of the wheel speed with the signal-to-signal intervalT_(n) thereof varies the intervals at a significant level. Therefore,the wheel speed derived based on the sensor signal pulse input timingswill be varied significantly. As a result, the wheel acceleration anddeceleration to be derived based on the sensor signal pulse inputtimings and relative to the wheel speed varies significantly.

Here, the controller unit 202 controls operation of the electric motor88 for controlling operation of the fluid pump 90. The electric motor 88is adapted to be driven when the wheel deceleration exceeds a givendeceleration threshold, e.g. -0.1G and thus the anti-skid control systembecome active. Therefore, even when the derived wheel decelerationexceeds the given threshold at a moment as shown in FIG. 19 due to errorin the wheel sensor signal, a driver signal is apt to be fed to theelectric motor 88 to drive the latter.

In order to prevent the electric motor from be operated in error due tofluctuation of the wheel sensor signal, the preferred embodiment of theanti-skid control system is provided with the wheel speed derivingroutine and the wheel acceleration and deceleration deriving routine atthe blocks 5004 of the output calculation program of FIG. 16 and theblock 2040 of the main routine of the main program of FIG. 15. FIG. 20shows the wheel speed V_(w) deriving routine. In the wheel speedderiving routine, the signal-to-signal interval T_(n) derived in any oneof the blocks 2026, 2030 and 2038 of the main routine of FIG. 15 is readat a block 5004-1. The wheel speed V_(w) is calculated from thesignal-to-signal interval T_(n) at a block 5004-2. The result of wheelspeed calculation is output at a block 5004-3 in the form of wheel speeddata indicative of the derived wheel speed value V_(w).

FIG. 21 shows a flowchart of the wheel acceleration and decelerationderiving program. As in the wheel speed deriving routine set forthabove, the signal-to-signal interval T_(n) derived at the blocks 2026,2030 and 2038 is read out at a block 2040-1. Based on thesignal-to-signal interval T_(n) and input timing data stored in thememory block 240, the wheel acceleration a_(w) is calculated accordingto the equation (1), at a block 2040-2. After this, the wheel speed dataderived by the wheel speed deriving routine is read out at a block2040-3. The read wheel speed is compared with the given threshold V_(th)which will be hereafter referring to as "wheel speed threshold", at ablock 2040-4.

When the wheel speed V_(w) is lower than the wheel speed thresholdV_(th) when check at the block 2040-4, the wheel acceleration anddeceleration a_(w) derived at the block 2040-2 is replaced with apredetermined fixed value a_(set) at a block 2040-5. Otherwise, thewheel acceleration and deceleration as derived at the block 2040-2 isoutput at a block 2040-6. Similarly, after replacing the derived wheelacceleration and deceleration a_(w) with the predetermined fixed valuea_(set), the replaced fixed value a_(set) is output as the wheelacceleration and deceleration indicative data a_(w) at the block 2040-6.

It should be noted that the predetermined fixed value a_(set) to whichthe output value of the wheel accleration and deceleration derivingroutine is set, is determined to be lower than the decelerationthreshold at which the anti-sid system is triggered, so that theanti-skid control system will remain inoperative as long as the wheelspeed remains lower than the wheel speed threshold. As a result, thefluid pump 90 will not be driven by the electric motor 88 as the driversignal to the latter may not be input.

In the embodiment shown, the predetermined fixed value a_(set) is set at0G, as shown in FIG. 22. Therefore, in the wheel speed range lower thanthe wheel speed threshold V_(th), the output of the wheel accelerationand deceleration deriving routine will be remained at the constant valuei.e. 0G.

FIG. 23 shows a flowchart of a wheel acceleration and decelerationderiving routine modified from the flowchart of FIG. 21. In thismodification, the operating condition of the electric motor 88 ischecked so as to check if the wheel speed is decelerated and theanti-skid control system is in operation. This check is performed at ablock 2040-7 as an additional block to the flowchart of FIG. 21.Checking of the electric motor condition is performed subsequently tothe block 2040-4 for checking if the wheep speed V_(w) is higher thanthe wheel speed threshold V_(th), and occurs only when the wheel speedV_(w) lower than the wheel speed threshold V_(th) is detected.

In the block 2040-7, the motor condition is checked if it is driving ornot. When the motor 88 is not driven when checked at the block 2040-7,the wheel acceleration and deceleration derived at the block 2040-2 isreplaced with the predetermined fixed value a_(set) at the block 2040-5.On the other hand, when the motor is driven, then, the wheelacceleration and deceleration a_(w) derived at the block 2040-2 isoutput as is at the block 2040-6.

Therefore, assuming the motor remains operational until a time p₂ afterthe wheel speed V_(w) drops lower than the wheel speed threshold V_(th)at a time p₁ ', the wheel acceleration and deceleration derived at theblock 2040-2 will be output without being replaced with the fixed valuea_(set) until the time p₂. After the time p₂ and in response toterminating of the motor operation, the wheel acceleration anddeceleration derived at the block 2040-2 is replaced with the fixedvalue a_(set) and thus the fixed value, i.e. 0G, is output.

FIG. 25 shows another embodiment of the controller unit 202 in thepreferred embodiment of the anti-skid control system according to thepresent invention. In practice, the circuit shown in FIG. 31 performsthe same procedure in controlling the actuator 16 and each block of thecircuit performs operations substantially corresponding to thoseperformed by the foregoing computer flowchart.

In FIG. 25, the wheel speed sensor 10 is connected to a shaping circuit260 provided in the controller unit 202. The shaping circuit 260produces the rectangular sensor pulses having a pulse interval inverselyproportional to the wheel speed V_(w). The sensor pulse output from theshaping circuit 260 is fed to a pulse pre-scaler 262 which counts thesensor pulses to produce a sample command for sampling input timing whenthe counter value reaches a predetermined value. The predetermined valueto be compared with the counter value in the pulse pre-scaler 262 isdetermined such that the intervals between the pairs of three successivesample commands will be sufficiently different to allow calculation ofthe wheel acceleration and deceleration rate.

The sample command is fed to a flag generator 264. The flag generator264 is responsive to the sample command to produce a flag signal. Theflag signal of the flag generator 264 is fed to a flag counter 266 whichis adapted to count the flag signals and output a counter signal havinga value representative of its counter value.

At the same time, the sample command of the pulse pre-scaler 262 is fedto a latching circuit 268 which is adapted to latch the signal value ofa clock counter signal from a clock counter 267 counting the clock pulseoutput by a clock generator 11. The latched value of the clock countersignal is representative of the input timing of the sensor pulse whichactivates the pulse pre-scaler 262 to produce the sample command. Thelatching circuit 268 sends the input timing indicative signal having avalue corresponding to the latched clock counter signal value, to amemory controller 274. The memory controller 274 is responsive to amemory command input from an interrupt processing circuit 272 which inturn is responsive to the flag counter signal to issue a memory commandwhich activates the memory controller 274 to transfer the input timingindicative signal from the latching circuit 268 to a memory area 276.The memory 276 sends the stored input timing indicative signal to asample controller 270 whenever the input timing signal valuecorresponding to the latched value of the latching circuit 268 iswritten therein. The sample controller 270 performs operationssubstantially corresponding to that performed in the blocks 2008, 2010,2012, 2032 and 2034 in FIG. 15, i.e. it determines number of sensorpulses in each group to be ignored. The sample controller 270 outputs apulse number indicative signal to the pulse pre-scaler 262, which pulsenumber indicative signal has a value approximating the predeterminedvalue to be compared with the counter value in the pulse pre-scaler 262.

The memory 276 also feeds the stored input timing indicative signal to awheel acceleration and deceleration calculation circuit 278 and a pulseinterval calculation circuit 280. The wheel acceleration anddeceleration calculation circuit 278 first calculates a pulse intervaldifference between pairs of three successively sampled sensor pulses.The obtained pulse interval difference is compared with a referencevalue so as to distinguish if the pulse interval difference is greatenough to allow calculation of the wheel acceleration and decelerationa_(w). If the obtained pulse interval difference is greater than thereference value, then the wheel acceleration and decelerationcalculation circuit 278 performs calculations of the wheel accelerationand deceleration according to the foregoing formula (1). If the obtainedpulse interval difference is smaller than the reference value, the wheelacceleration and deceleration calculation circuit 278 shifts theoperational mode thereof so as to achieve a pulse interval differencelarge enough to permit the wheel acceleration and decelerationcalculation. On the other hand, the pulse interval calculation circuit280 performs calculations to obtain the pulse interval between thecurrent pulse and the immediate preceding pulse and sends a pulseinterval indicative signal to a memory 282. The memory 282 sends astored pulse interval indicative signal to a wheel speed calculationcircuit 284 which is associated with a 10 ms timer 292. The 10 ms timer292 produces a timer signal every 10 ms to activate the wheel speedcalculation circuit 284. The wheel speed calculation circuit 284 isresponsive to the timer signal to perform calculation of the wheel speedV_(w) by calculating the reciprocal value of the pulse intervalindicative signal from the memory 282. The wheel speed calculationcircuit 284 thus produces a wheel speed indicative signal to be fed to atarget wheel speed calculation circuit 288 and to a slip ratecalculation circuit 290 which is also associated with the 10 ms timer tobe activated by the timer signal every 10 ms.

The target wheel speed calculation circuit 288 is adapted to detect thewheel speed V_(w) at which the wheel acceleration and deceleration a_(w)calculated by the wheel acceleration and deceleration calculatingcircuit 278 exceeds than a predetermined deceleration rate -b. Thetarget wheel speed calculation circuit 288 measures the interval betweentimes at which the wheel deceleration exceeds the predetermineddeceleration value. Based on the wheel speed at the foregoing times andthe measured period of time, the target wheel speed calculation circuit288 derives a decelerating ratio of the wheel speed to produce a targetwheel speed indicative signal. The target wheel indicative signal of thetarget wheel speed calculation circuit 288 and the wheel speedindicative signal from the wheel speed calculation circuit 284 are fedto a slip rate calculation circuit 290.

The slip rate calculation circuit 290 is also responsive to the timersignal from the 10 ms timer 292 to perform calculation of the slip rateR based on the wheel speed indicative signal from the wheel speedcalculation circuit 284 and the target wheel speed calculation circuit288, in accordance with the formula (2).

The slip rate calculation circuit 290 and the wheel acceleration anddeceleration calculation circuit 278 are connected to an output unit 294to feed the acceleration and deceleration indicative signal and the sliprate control signal thereto. The output unit 294 determines theoperation mode of the actuator 16 based on the wheel acceleration anddeceleration indicative signal value and the slip rate indicative signalvalue according to the table of FIG. 26. The output unit 294 thusproduces the inlet and outlet signals EV and AV with a combination ofsignal levels corresponding to the selected operation mode of theactuator.

On the other hand, the wheel speed calculation circuit 284 is alsoconnected to the flag counter 266 to feed a decrementing signal wheneverthe calculation of the wheel speed is completed and so decrement thecounter value of the flag counter by 1. The flag counter 266 is alsoconnected to a comparator 295 which is adapted to compare the countervalue of the flag counter with a reference value, e.g. 2. When thecounter value of the flag counter 266 is greater than or equal to thereference value, the comparator 295 outputs a comparator signal to anoverflow detector 296. The overflow detector 296 is responsive to thecomparator signal to feed a sample mode shifting command to be fed tothe pulse pre-scaler 262 to shift the sample mode to increase the numberof the sensor pulses in each sample group.

On the other hand, the clock counter 267 is connected to an overflowflag generator 297 which detects when the counter value reaches the fullcount of the clock counter to produce an overflow flag signal. Theoverflow flag signal of the overflow flag generator 297 is fed to anoverflow flag counter 298 which is adapted to count the overflow flagsignals and send an overflow counter value indicative signal to ajudgment circuit 299. The judgment circuit 299 compares the overflowcounter indicative signal value with a reference value e.g. 2. Thejudgment circuit 299 produces a reset signal when the overflow counterindicative signal value is equal to or greater than the reference value.The reset signal resets the wheel acceleration and decelerationcalculation circuit 278 and the wheel speed calculation circuit 284 tozero. On the other hand, the overflow flag counter is connected to thewheel speed calculation circuit 284 and is responsive to thedecrementing signal output from the wheel speed calculation circuit asset forth above to be reset in response to the decrementing signal.

FIG. 26 shows the detailed structure of the wheel acceleration anddeceleration calculation circuit 278 in accordance with the preferredembodiment of the present invention. In this embodiment, the wheelacceleration and the deceleration calculation circuit 278 is associatedwith the wheel speed calculation circuit 284 in order to perform outputcontrol similar to that disclosed with respect to the first embodimentand as described with reference to FIG. 21.

The wheel acceleration and deceleration calculation circuit 278comprises an arithmetic circuit 278-1 and a comparator circuit 287-2.The arithmetic is connected to the memory 276 to receive therefrom theinput timing data of the sensor signal pulses. The arithmetic circuit278-1 performs an arithmetic operation according to the equation (1) toderive the wheel acceleration and deceleration value a_(w) based on theinput timing data from the memory 276. On the other hand, the comparatorcircuit 278-2 is connected to the wheel speed calculation circuit 284 toreceive therefrom the wheel speed indicative signal having a valuerepresentative of the wheel speed derived by the wheel speed calculationcircuit. The comparator circuit 278-2 compares the wheel speedindicative signal value with a reference value representative of thewheel speed threshold V_(th). The comparator circuit 278-2 is adapted tofeed a command to the arithmetic circuit 278-1 for disabling same tooutput the wheel acceleration and deceleration indicative signal havinga value representative of the derived wheel acceleration anddeceleration. Then, the arithmetic circuit 278-1 outputs the fixed valueof the signal as the wheel acceleration and deceleration indicativesignal. As set forth with respect to the first embodiment, the fixedvalue to be output at this condition is indicative of the wheelacceleration and deceleration 0G, for example.

FIG. 27 shows a modification of the wheel acceleration and decelerationcalculation circuit of FIG. 26. This modification is adapted to performan operation substantially corresponding to that performed by theflowchart of FIG. 23.

In FIG. 27, the wheel acceleration and deceleration calculation circuit278 comprises an arithmetic circuit 287-1' which receives the inputtiming data from the memory 276 and derives a wheel acceleration anddeceleration based on the input timing data of the sensor signal pulses.The arithmetic circuit 287-1' is connected to a selector switch 278-3.The selector switch 287-3 is also connected to a memory 278-2' whichstores a predetermined fixed value a_(set), e.g. a value correspondingto or representative of 0G. The selector switch 278-3 is furtherconnected to a low wheel speed detector 278-4 which detects the wheelspeed V_(w) derived by the wheel speed calculation circuit 284 lowerthan the predetermined wheel speed threshold V_(th). The low wheel speeddetector 278-4 produces a detector signal when the derived wheel speedV_(w) is lower than the wheel speed threshold.

The selector switch 278-3 is normally biased to a position connectingthe arithmetic circuit 278-1' to the output unit 294 therethrough so asto feed the wheel acceleration and deceleration indicative signal havinga value corresponding to the derived wheel acceleration and decelerationvalue. The selector switch 278-3 is responsive to the detector signalfrom the low wheel speed detector 278-4 to switch the position toconnect the memory 278-2' to the output unit 294 to feed the storedfixed value a_(set) to the latter as the wheel acceleration anddeceleration indicative signal.

The selector switch 278-3 is further connected to a disabling circuit278-5 which is, in turn, connected to a motor control circuit 88-1. Themotor control circuit 88-1 is adapted to feed a driving signal while themotor is in operation or driving to the disabling circuit 278-5. Thedisabling circuit 278-5 is responsive to the driving signal from themotor control circuit to output a disable signal to the selector switch278-3. The selector switch 278-3 is disabled by being shifted from thenormal position to the shifted position as long as the disable signal ispresent. Therefore, even when the detector signal is input, the selectorswitch 278-3 maintains its normal position to feed the wheelacceleration and deceleration indicative signal having a valuerepresentative of the wheel acceleration and deceleration derived by thearithmetic circuit 278-1' to the output unit 294, when the disablesignal is input thereto.

Therefore, according to the embodiment shown, the wheel acceleration anddeceleration data indicative of a wheel deceleration greater than a setvalue at which anti-skid control operations are triggered will not beproduced while the wheel speed remains within a speed range in whichwheel sensor signal fluctuations are significant since the wheelacceleration and deceleration is set to a fixed constant value which islower than the set value.

Accordingly, the present invention fulfills all of the objects andadvantages sought.

What is claimed is:
 1. An anti-skid brake control system for an automotive vehicle comprising:a hydraulic brake circuit including a wheel cylinder for applying braking pressure to a vehicle wheel; a pressure control valve disposed within said hydraulic brake circuit for increasing the fluid pressure in said wheel cylinder in a first position thereof, decreasing the fluid pressure in a second position thereof and holding the fluid pressure at a constant value in a third position thereof; first means for detecting wheel speed and producing a first signal having a value representative of the wheel speed; a second means for detecting wheel acceleration and producing a second signal having a value representative of the wheel acceleration; a third means, responsive to said first and second signals, for deriving a control signal for selecting one of said first, second and third positions of said pressure control valve, said third means being responsive to said second signal value decreasing less than a given deceleration threshold to initiate derivation of said control signal for selecting said third position, said deceleration threshold serving as a criteria for initiating anti-skid brake control; a fourth means receiving said first signal for producing a command to replace said second signal value by a predetermined value which is representative of a lesser deceleration value than said deceleration threshold when said first signal value becomes less than a given wheel speed threshold which is representative of a low wheel speed criteria; and a fifth means for detecting the presence of said control signal selecting said third position for disabling said fourth means when anti-skid control is performed.
 2. The anti-skid brake control system as set forth in claim 1, wherein said fourth means includes a memory for storing said predetermined value.
 3. The anti-skid brake control system as set forth in claim 2, further comprising a selector switch means connected to said second means and said memory in said fourth means for selecting one of said second signal and a signal from said memory indicative of said predetermined value, said selector switch being normally in a first position in which said second signal is selected and responsive to said command to be shifted to a second position in which said predetermined value indicative signal from said memory is selected.
 4. A method for controlling a hydraulic brake system for an automotive vehicle to prevent a vehicle wheel from skidding, comprising the steps of:detecting wheel speed and producing a first signal indicative of the detected wheel speed; detecting wheel acceleration and producing a second signal indicative of the detected wheel acceleration; controlling fluid pressure applied to a wheel cylinder in different operational modes of a hydraulic brake circuit by increasing, decreasing or maintaining constant the fluid pressure therein in accordance with said first and second signal values; switching the operation mode of said hydraulic brake circuit from increasing of said fluid pressure to maintaining fluid pressure constant when said second signal representative of the detected wheel acceleration has a value below a predetermined deceleration threshold; detecting when said first signal has a value smaller than a given wheel speed threshold representative of a low wheel speed criteria and at that time, producing a signal indicative of a predetermined fixed value corresponding to an assumed wheel deceleration value less than a given wheel deceleration threshold; outputting said fixed value as a replacement for said second signal, while said first signal value is smaller than said given wheel speed threshold; and disabling the outputting of said predetermined fixed value indicative signal and enabling output of said second signal when anti-skid control operation is performed.
 5. The method as set forth in claim 4, further comprising steps of producing sensor signal pulses corresponding to the angular speed of the vehicle wheel, producing timer signals, sampling said timer signals in response to every sensor signal pulse and deriving said wheel acceleration from said sampled timer signal. 